So on air loads - no problem here... Or milion ribs . On shear due to torsional loads picture is not as happy

I do not need it now. But may run numbers for others.

Well, that is a relief. In a slow airplane a fiberglass skin with no foam is not necessarily a limiting factor. I know in my 268 knot Vd it certainly is.

Billski will you be able to provide shear loads due to wing torsional loads ? I can build few sections and load them too.

Not so simple as you might think. I could do just about any airfoil, but it is a lot of data entry (a failure prone process for this for this slightly dyslexic engineer) and other fuss for me who would rather be out making wingtip lenses. Tell you what, I have the Riblett 37A315 documented, which should be similar to other wings, but not identical.

tau = T*r/J

M = q*Cm*c*S

at 120 knots, q is 49 lb/ft^2, Cm tends to run around -0.05, c we have been talking about is 4ft, and S is 120ft^2. Since we are looking for the pitching moment of one wing at the root, we divide by two. Since we have a real wing with lift falling off towards the tip we multiply by Pi/4. 463 ft-lb, which is also 5552 in-lb.

Let's remember that we also usually have a somewhat lower g limit and max airspeed for flaps deployed. Yeah, lower q, but Cm can go to -1.0, so that case is worht running too.

Radius for tau calc varies widely and is based at the centroid of twist for the foil with the spar in it. My calcs all run wings that end at about .75c because my wings have flaps and ailerons filling the trailing edge, so my skin centroid is about where the main spar is, at .38c, and about 0.02c above the chord line. Since pressures and panel stresses are biggest up hear the leading edge, let's get r for upper skin at 0.15c. works out to 0.236*c = 11.35 inches.

J is kind of neat, I have curve fit it as 0.0886*c^3*t = 17.6 in^4 for a 0.018 (one ply 18 oz BIAX). Plug it into the equation and

tau = 3580 psi . Hmm or about 64.4 lb per linear each skin for a 0.018" skin.

Which has to be superimposed upon the inflation stresses and the bending stresses. We still do not have bending stresses, so let's do them.

Time for another estimator - The bending strain of the wing spar caps at max g pretty much follows the bending strain of the main spar because it is bonded to it. The skin then sees somewhere between 1/4 and 1/3 of the failure strain of the spar cap materials. Why so small? FOS is 2.0, so we get to half right away. Then when we make make strength under bending and shear at something approaching min weight, we end up beefing the caps substantially. Let's call it 1/3 of failure strain. Oh, we are watching the skin at about 0.15c, which puts the skins about 3/4 of the height of the caps. So, let's make it 1/3*3/4 = 1/4 of first fiber failure in the caps.

How much is that in the way of loads? Well, if you can impose strains in certain axes, that is the most straight forward solver. Hmm, E-glass at +/-45 has E of about 1.8Mpsi, T300 at +/-45 is about 8.4Mpsi, strain times thickness times E is load. Unidirectional E-glass first fiber failure strain is about 70kpsi and E is about 4.4Mpsi, so failure strain is 1.6%, and the wing skins will see tensile or compressive strains of about 0.4% strain or about 130 pounds per linear inch in 0.018" thick BIAX. Unidirectional T300 modulus is 181GPa and strengths are 1500MPa, so first fiber strain at failure is about 0.8%, so the wing is seeing about 0.2% strain or about 420 pounds per linear inch in 0.008" thick cloth laminate. Work with other materials and orientation, and you can now figure out what is going on for the models.

So now we have a way to look at inflation, torsion, and bending. So let's look at the total stress state. Top skin in E-glass x, y and shear stresses:

- X direction has about 1/4 of strength or 18,500 psi compressive plus whatever the inflation stresses are;
- Y direction has the inflation stresses;
- Shear along the edges are about 3580 psi.

Each of these produces strains and is run against failure criteria. Or you decide on laminate thickness and put in loads on the edges of the panel and airload on the skin and let the FEA run.

Billski